Re: [syzbot] [input?] KASAN: slab-use-after-free Read in input_dev_uevent

[Dmitry Torokhov]

Hi Maxime,

On Wed, Aug 23, 2023 at 03:16:02PM +0200, Maxime Ripard wrote:
> Hi Dmitry,
> 
> On Wed, Aug 23, 2023 at 05:51:00AM -0700, Dmitry Torokhov wrote:
> > On Wed, Aug 23, 2023 at 09:44:22AM +0200, Maxime Ripard wrote:
> > > On Tue, Aug 22, 2023 at 08:57:41AM -0700, Rahul Rameshbabu wrote:
> > > > On Tue, 22 Aug, 2023 11:12:28 +0200 Maxime Ripard <mripard@xxxxxxxxxx> wrote:
> > > > > Hi,
> > > > >
> > > > > So, we discussed it this morning with Benjamin, and I think the culprit
> > > > > is that the uclogic driver will allocate a char array with devm_kzalloc
> > > > > in uclogic_input_configured()
> > > > > (https://elixir.bootlin.com/linux/latest/source/drivers/hid/hid-uclogic-core.c#L149),
> > > > > and will assign input_dev->name to that pointer.
> > > > >
> > > > > When the device is removed, the devm-allocated array is freed, and the
> > > > > input framework will send a uevent in input_dev_uevent() using the
> > > > > input_dev->name field:
> > > > >
> > > > > https://elixir.bootlin.com/linux/latest/source/drivers/input/input.c#L1688
> > > > >
> > > > > So it's a classic dangling pointer situation.
> > > > >
> > > > > And even though it was revealed by that patch, I think the issue is
> > > > > unrelated. The fundamental issue seems to be that the usage of devm in
> > > > > that situation is wrong.
> > > > >
> > > > > input_dev->name is accessed by input_dev_uevent, which for KOBJ_UNBIND
> > > > > and KOBJ_REMOVE will be called after remove.
> > > > >
> > > > > For example, in __device_release_driver() (with the driver remove hook
> > > > > being called in device_remove() and devres_release_all() being called in
> > > > > device_unbind_cleanup()):
> > > > > https://elixir.bootlin.com/linux/latest/source/drivers/base/dd.c#L1278
> > > > >
> > > > > So, it looks to me that, with or without the patch we merged recently,
> > > > > the core has always sent uevent after device-managed resources were
> > > > > freed. Thus, the uclogic (and any other input driver) was wrong in
> > > > > allocating its input_dev name with devm_kzalloc (or the phys and uniq
> > > > > fields in that struct).
> > > > >
> > > > > Note that freeing input_dev->name in remove would have been just as bad.
> > > > >
> > > > > Looking at the code quickly, at least hid-playstation,
> > > > > hid-nvidia-shield, hid-logitech-hidpp, mms114 and tsc200x seem to be
> > > > > affected by the same issue.
> > > > 
> > > > I agree with this analysis overall. At least in hid-nvidia-shield, I can
> > > > not use devm for allocating the input name string and explicitly free it
> > > > after calling input_unregister_device. In this scenario, the name string
> > > > would have been freed explicitly after input_put_device was called
> > > > (since the input device is not devres managed). input_put_device would
> > > > drop the reference count to zero and the device would be cleaned up at
> > > > that point triggering KOBJ_REMOVE and firing off that final
> > > > input_dev_uevent.
> > > > 
> > > > I think this can be done for a number of the drivers as a workaround
> > > > till this issue is properly resolved. If this seems appropriate, I can
> > > > send out a series later in the day. This is just a workaround till the
> > > > discussion below converges (which I am interested in).
> > > 
> > > I'm sorry, I don't know the input framework well enough to understand
> > > what you had in mind exactly. Could you send a patch with your
> > > suggestion for the hid-nvidia-shield so we can discuss this further?
> > > 
> > > That being said, I think that the current design around name, phys and
> > > uniq is fairly treacherous to drivers and we should aim for a solution
> > > that prevents that issue from being possible at all.
> > > 
> > > I was inclined to go for a char array for each to get rid of the pointer
> > > entirely, but Benjamin raised some concerns over the structure size so
> > > it's probably not a great solution.
> > 
> > I think everything is much simpler, with uclogic driver being in the
> > wrong here: devm resource needs to be attached to the right device
> > (instance of HID) rather than to the input device itself (which should
> > never have any driver resources attached since it never has a driver).
> > Something like this:
> > 
> > diff --git a/drivers/hid/hid-uclogic-core.c b/drivers/hid/hid-uclogic-core.c
> > index f67835f9ed4c..f234a7c97360 100644
> > --- a/drivers/hid/hid-uclogic-core.c
> > +++ b/drivers/hid/hid-uclogic-core.c
> > @@ -148,7 +148,7 @@ static int uclogic_input_configured(struct hid_device *hdev,
> >  
> >  	if (suffix) {
> >  		len = strlen(hdev->name) + 2 + strlen(suffix);
> > -		name = devm_kzalloc(&hi->input->dev, len, GFP_KERNEL);
> > +		name = devm_kzalloc(&hdev->dev, len, GFP_KERNEL);
> >  		if (name) {
> >  			snprintf(name, len, "%s %s", hdev->name, suffix);
> >  			hi->input->name = name;
> > 
> > In general, drivers should attach devm resources they allocate to the
> > instance of device they are binding to, and nothing else.
> 
> I'm not sure that's enough unfortunately. The fundamental issue here
> seems to be that input_dev_uevent follows a pointer that can be
> allocated by the driver, and will be free'd before the last call to
> input_dev_uevent.

Yes, this is a fundamental property of C pointers - you should not free
them before exiting last code section that may reference them. For input
devices it means that pointers should not be freed until after
input_unregister_device() is called.

I.e. you have sequence like this:

	driver_data = kzalloc(...);
	driver_data->input_name = kstrdup(...);
	driver_data->input_phys = kstrdup(...);
	input = input_allocate_device();
	input->name = driver_data->input_name;
	input->phys = driver_data->input_phys;
	input_register_device(input);
	...

	input_unregister_device(input);
	kfree(driver_data->input_name);
	kfree(driver_data->input_phys);
	kfree(driver_data);


devm typically helps with resources being freed at the right time, but
for that the managed resources should be attached to the *correct
device*, with correct device being one the driver is binding to, not any
random device structure.

> 
> And I think that's true for both devices here

Yes, it looks like the shield is also using wrong device.

Thanks.

-- 
Dmitry